1. Field of the Invention
The present invention relates to a supporting means for an ultrasonic motor for generating driving force by the use of elastic vibrations excited by anti-piezoelectric effects of a piezoelectric member.
2. Description of the Related Arts
In recent years, an ultrasonic motor is watched where elastic vibrations are excited in a vibration member with the use of a piezoelectric member such as peizoelectric ceramic or the like so as to make them as driving force.
A conventional ultrasonic motor will be described hereinafter in detail with reference to drawings.
FIG. 1 is a sectional view showing the construction of the conventional ultrasonic motor. Referring now to FIG. 1, reference numeral 1 is a ring shaped elastic member composed of material such as metal, ceramic or the like, reference numeral 2 is a ring shaped piezoelectric member, reference numeral 3 is a stator composed of an elastic member 1 and a piezoelectric member 2 stuck to each other. The elastic body 1 has a projection portion 4 of the same diameter disposed on it. Reference numeral 5 is a disk-shaped support member of a thin plate so as to support the stator. Reference numeral 6 is a rotor composed of a construction similar to that of the stator 4. Reference numeral 7 indicates two bearings each having a rotary shaft 8 penetrated through a central hole of the bearing 7. Reference numeral 9 is a housing, reference numeral 10 is a control circuit for driving, controlling the stator 3.
In FIG. 1, when an alternating current voltage is applied on a piezoelectric member 2 from a control circuit, standing waves of a primary bending vibration mode or more in the radial direction, and a tertiary bending vibration mode or more in the peripheral direction, having a vibration displacement distribution in the radial direction as shown in FIG. 2 are excited in the stator 3. In order to secure it in a position where the vibration loss is small, a disk-shaped support member 5 is disposed near a neutral face of the stator 3. The stator is secured to the housing 9 at the inner peripheral portion thereof. When two standing waves having a given phase difference are excited in the ring shaped stator 3, progressing waves of bending vibrations which are progressing in the peripheral direction are excited in the stator 3. A rotor 6 which is disposed in pressure contact with the projecting portion 4 of the stator 3 is driven by a frictional force created by the progressing waves and the rotary shaft 8 is mechanically coupled to the rotor 6. As the shaft is retained by two bearings 7, the rotary shaft 8 is rotated.
FIG. 3 is a view indicating one example of a driving electrode construction of a piezoelectric member 2 used for an ultrasonic motor of FIG. 1. FIG. 3 indicates an electrode construction where nine elastic waves are excited in the peripheral direction. Reference characters A and B respectively designate groups of electrodes each corresponding to one-half of a wavelength. Reference character C designates an electrode corresponding to three fourths of a wavelength, reference character D designates an electrode corresponding to one fourth of a wavelength. Electrodes C and D are provided to cause a phase difference of one fourth of a wavelength (=90 degrees) in position between the electrode groups A and B. Small electrode portions adjacent within the electrodes A and B are polarized in a thickness direction opposite to each other. A face at which the piezoelectric member 2 is bonded to the elastic member is with respect to an elastic member 1 of a piezoelectric member 2 is opposite to a face shown in FIG. 3. An electrode is a non-segmented electrode. At a driving operation time, the electrode groups A and B are respectively short-circuited and used as shown with the oblique lines in FIG. 3.
The conventional ultrasonic motor constructed as described hereinabove will be described hereinafter in its operation. Voltages V1 and V2 represented by equations (1), (2) are respectively applied upon the groups of electrodes A and B of the piezoelectric member 2 of the ultrasonic motor. EQU V.sub.1 =V.sub.0 .times.sin (.omega. t) (1) EQU V.sub.2 =V.sub.0 .times.cos (.omega. t) (2)
where V.sub.0 is an instantaneous value of the voltage, .omega. is an angular frequency, t is time.
Therefore, the progressive waves of the bending vibrations in the circumferential direction represented by an equation (3) are excited in the stator 3. ##EQU1## wherein .epsilon. is an amplitude value of the bending vibrations, .epsilon..sub.0 is an instantaneous value of the bending vibrations, k is a ratio (2.pi./.lambda.), .lambda. is a wavelength, x is a position.
FIG. 4 indicates the movement of an elliptic track of 2 w in vertical axis and 2 u in lateral axis to be obtained by the excitation of the progressive wave at a point A on the surface of the stator 3. A rotor 6 set pressed onto the stator 3 comes near a vertex of the elliptic track 11 so as to move at a speed v of an equation (4) in a direction opposite to the wave progressive direction by a frictional force. EQU v=.omega..times.u (4)
In the construction of the conventional ultrasonic motor, a disc-shaped support member 5 has to be formed by a method of splicing it with the stator 3 or to be worked at the same time. At the splicing operation, an uneven vibration distribution is likely to be caused because of inequalities in mechanical strength at the splicing portion. At an integral working operation, lower mass production is caused because of working precision and a working operation to be effected from both the faces of the stator 3. As shown in FIG. 1, the conventional construction has a problem in that the vibrations of the stator are large influenced with respect to change in motor shape and pressure force of the rotor so that the vibration displacement distribution in the radial direction shown in FIG. 2 cannot be retained, thus resulting in considerable output reduction, because the projection portion 4 has the same diameter as that of the stator 3 in a maximum position of the vibration change quantity without fail.